Wraps are not considered 2nds?

[Sean Dezoysa]

Here's what I (think I) know:
- Wrap mortgages include the terms of the original mortgage in them
- They are designed to "wrap around" the existing mortgage terms, and add some new ones
- Investors often use them to arbitrage payments between underlying mortgage and what is owed to them

Here's what I don't get
- A wrap is consider a junior lien yet is not considered a 2nd. It is considered a first, even though the "original first" (the underlying lien) is not paid off.

Why is that?